Most of organisms acquire energy necessary for life activity by respiration. In higher organisms, carbohydrates, proteins, and aliphatic acids are degraded into acetyl-CoA by the glycolytic pathway and the .beta.-oxidation in cytoplasm, and acetyl-CoA is degraded by the citric acid cycle in mitochondria. The resulting energy is saved as reducing power of NADH and FADH.sub.2. Finally, NADH is completely oxidized to water by the subsequent electron transport system that is present on mitochondrial inner membranes, and a proton concentration gradient is formed in a coupled manner to the oxidation, and serves as driving force of the ATP synthesis.
Since the bacterial respiratory chain generally comprises various functional enzyme complexes depending on species and growing circumstance, the energy conservation efficiency may vary to a great extent. For example, Escherichia coli contains at least two kinds of quinol oxidase, bo type and bd type, which function as terminal oxidases in the respiratory chain. When a wild-type strain carrying the enzymes of the both types, a mutant strain carrying only the bo type, and a mutant strain carrying only the bd type are compared as for growth yield observed in aerobic culture, the growth yield is the lowest in the mutant carrying only the bd type enzyme, and depends on the kind of the terminal oxidases and their energy conservation efficiency (Lecture Abstract for The Conference of The Society for Bioscience and Bioengineering, Japan, 1995, Subject No. 357).
Coryneform bacteria such as Brevibacterium lactofermentum and Brevibacterium flavum are gram-positive and aerobic bacteria that are industrially utilized for amino acid producers. Although terminal oxidases of the respiratory chain have been well investigated as for those of Proteobacteria, which is phylogenetically quite far from the coryneform bacteria, and those of Bacillus subtilis and the thermophilic Bacillus, which are also gram-positive bacteria like the coryneform bacteria but phylogenetically somewhat different from them, the electron transport system of respiratory chain in coryneform bacteria has not been investigated in detail. It is considered that it is important to elucidate the electron transport system of the respiratory chain, which is the key of the energy metabolism, in coryneform bacteria in view of collecting fundamental data for improving productivity of useful substances. Further, if enzymes involved in the electron transport system of the respiratory chain in coryneform bacteria and genes therefor are identified, they may be useful for, for example, creating strains with higher energy efficiency.
To date, it has been reported that the respiration of Brevibacterium lactofermentum is coupled to the proton transport, and it involves cytochromes a, b, and c (Kawahara, Y., et al. (1988) Agric. Biol. Chem., 52(8), 1979-1983). Cytochrome bd type quinol oxidase of Brevibacterium flavum has also been purified and characterized (Kusumoto, Sone and Sakamoto, "Respiratory Chain of Amino Acid Fermenting Bacterium, Brevibacterium flavum, and Characteristics of Its Cytochrome bd Type Menaquinol Oxidase", Abstracts of the 23th Symposium of Bioenergy Study Group, 1997). However, there has not been any report concerning the genes encoding cytochrome bd type quinol oxidase of coryneform bacteria.